Families of the victims of the 7 July suicide bombings today stepped up demands for an independent inquiry, as a long-awaited report into whether the atrocities could have been prevented revealed that the bombers' leader had crossed the radar of police and MI5 on eight occasions but had never been assessed as a threat.

The intelligence and security committee (ISC) cleared the police and agencies of any blame for failing to track Mohammad Sidique Khan and his right-hand man, Shehzad Tanweer, after they appeared as part of an investigation into a plot to detonate fertiliser bombs in the UK, almost 18 months before the 2005 attacks in London; the ISC said there was no evidence the pair planned to launch attacks themselves.

But it also listed a string of occasions, dating back to 1993, when versions of Khan's name or addresses connected to him were recorded by police or MI5, all but the earliest because of links to people being investigated over extremism.

In unprecedented detail, the committee also revealed a lack of co-operation between MI5 in London and police special branch in West Yorkshire. But it concluded that MI5 suffered at the time from a serious lack of resources.

The heavily censored, 102-page report revealed that in 2001 West Yorkshire police had videoed Khan at a training camp in the UK organised by two known extremists, an event described as a "significant lead" by the ISC. Images from the footage were shown to their sources but Khan was not identified.

The other pieces of information on Khan, held variously by West Yorkshire police, the Metropolitan police and MI5, were never connected before 7 July. It was "surprising" that MI5 said it had not identified Khan by then, given the amount of information held, the report said.

Rachel North, a survivor of the attacks, which killed 52 people, said she was "sad but not surprised" by the report. She described it as a "catalogue of excuses for MI5's narrow focus and failure of intelligence caused by failure of imagination and failure of co-operation [between police and MI5] at a critical time".

The report revealed that MI5 is not automatically informed when the police special branch receives intelligence about terrorism. It also said MI5 had no legal power to pass to the police all the intelligence it collects from counter-terrorism operations. But the committee concluded that, having looked at all the evidence in "considerable" detail, "we cannot criticise the judgments made by MI5 and the police on the information that they had and their priorities at the time".

Ten secure emails were exchanged between MI5 and West Yorkshire police referring to an individual now known to be Khan after the fertiliser bomb plotters had been arrested, the ISC discovered, but the force "did not find anything significant".

The chairman of the ISC, Kim Howells, said: "Our criticism at the time was mainly that MI5 acted on a need-to-know basis and there ought to have been a more complete dialogue between MI5 and special branch."

The report repeatedly asserted that MI5's lack of resources meant it could not have followed Khan and Tanweer after the fertiliser plotters had been arrested. Even if the service had had greater capacity, it would not have happened since they were not seen as a threat, it added.

It described it as "astounding" that in 2004 it was not possible to provide intelligence coverage for 52 "essential" targets, and MI5 could only provide reasonable coverage for 6% of the overall known threat. The MPs said that despite an increase in surveillance capability since then, the committee remained "concerned that not enough targets can be covered adequately".

The committee said that in the course of this second review of the intelligence it had uncovered new information that even the organisations involved had not connected together before.

Giving evidence in June 2007, the head of MI5 admitted it was "unsatisfactory" that even then the service was discovering new references to Khan.

It was not until after the 2005 attacks that a conversation in the car of Omar Khyam, leader of the fertiliser plotters, was fully transcribed and then revealed that Tanweer was present, praising the Madrid bombings.

The report said it was possible that a "facilitator", whose name is redacted, was involved in the 7 July bombings. The Guardian understands the individual is Rashid Rauf, a British-Pakistani believed to have died in an American drone strike on the Afghan border.

The former shadow home secretary, David Davis, said there was a clear need for a senior judge to lead an investigation. "There were errors which, if avoided, would have led to a prevention of this attack."

Gordon Brown said there was no evidence to support allegations about missed clues or ignored warnings and that an investigation into the fertiliser bombs understandably took precedence.